acces on NVME disk

Hello
I do not know if this is a bug in version 17.10 (artfuf) and probably earlier versions:

It is possible to access without any modification of the bios to the NVME disk when the boot takes place in LEGACY mode.

It is necessary to set the AHCI mode instead of the RAID mode in the bios to access the NVME disk when booting is in LEGACY mode

487027] ahci 0000: 00: 17.0: Found 1 remapped NVMe devices.
[2.487028] ahci 0000: 00: 17.0: Switch your BIOS from RAID to AHCI mode to use them.
[2.487047] ahci 0000: 00: 17.0: SSS flag set, Parallel Bus Scan Disabled

I do not understand this difference of treatment except to think that the bios does not manage in the same way the access to the NVME according to how to boot or that ubuntu is less demanding when a boot in mode LEGACY,